System and method of watermarking a video signal and extracting the watermarking from a video signal

ABSTRACT

This method comprises the following steps of applying a watermarking function to motion vectors calculated by estimation of movement between images of a video signal, followed by a step of generating the watermarked video signal by compensating movement with the aid of the watermarked motion vectors. The watermarking function is applied by marking the coordinates of the selected motion vector in a reference space, certain portions of this space constituting a first zone and the other portions of this space constituting a second zone complementary to the first, assigning a binary value to each of the two zones, and, if necessary, modifying the coordinates of the selected motion vector so that it is in the zone whose binary value corresponds to a bit of a marking key associated with the selected motion vector.

FIELD OF THE INVENTION

The present invention relates to a method of watermarking a videosignal, a corresponding system, and a computer readable data medium forstoring a signal watermarked by this method. It also relates to a methodof extracting the watermarking from a video signal and a system forimplementing this method.

To be more precise, the invention relates to a method of watermarking avideo signal by applying a watermarking function to motion vectorscalculated by motion estimation between images of the video signal, themethod being of the type comprising the following steps:

-   -   applying the watermarking function to at least some of the        calculated motion vectors; and    -   generating the watermarked video signal by motion compensation        with the aid of the watermarked motion vectors,        the watermarking function being applied with the aid of a binary        marking key, each bit of which is associated with at least one        selected motion vector.

BACKGROUND OF THE INVENTION

A method of the above kind is described in the paper by F. Jordan, M.Kutter, and T. Ebrahimi, “Proposal of a watermarking technique forhiding/retrieving data in compressed and decompressed video”, ISO/IECdocument JTC1/SC29/WG11/MPEG97/M2281, of July 1997. In that document,the watermarking function is applied with the aid of a marking key. Thisis a binary word of 16 or 32 bits inserted into the motion vectors ofthe video signal, which motion vectors are obtained with the aid of anMPEG4 encoder. The number of motion vectors selected is the same as thenumber of bits in the marking key, i.e. 16 or 32 motion vectors areselected. Then, for each of the vectors selected, the corresponding bitof the marking key is inserted into one of the components of the motionvector, for example the vertical component, modifying its parity.

Unfortunately, that solution is not very robust, since the slightestattack can transform an even ordinate into an odd ordinate andvice-versa. Here, the term “attack” means malicious attacks and alsonon-malicious attacks such as compression or changing the spatial or thetemporal format of the video signal. Moreover, by always modifying thesame predetermined component of the selected motion vectors, for examplethe vertical component, that solution runs the risk of making thewatermarking more visible in the video signal.

SUMMARY OF THE INVENTION

The invention aims to remedy those drawbacks by providing a robustmethod of watermarking a video signal that improves watermarkingperformance in terms of invisibility and in which the watermarking iscarried by the motion vectors.

To this end, the invention consists in a method of the above-specifiedtype for watermarking a video signal, wherein, in order to apply thewatermarking function, the method further comprises the following steps:

-   -   marking the coordinates of the selected motion vector in a space        including a reference partition, certain portions of this space        constituting a first zone and the other portions of this space        constituting a second zone, complementary to the first;    -   assigning a binary value to each of the two zones; and    -   where necessary, modifying the coordinates of the selected        motion vector so that it is in the zone whose binary value        corresponds to the bit of the marking key with which the        selected motion vector is associated.

With this approach, greater diversity in the modification of theselected motion vectors can be envisaged, which improves the quality ofthe watermarking in terms of invisibility. Robustness is also improvedsince the watermarking leads to modifying the components of the selectedmotion vectors by a geometrical transformation and not solely by achange of parity.

A method of the invention may further comprise the feature whereby thereference partition is a reference grid comprising blocks withpredefined dimensions, each block comprising a first zone and a secondzone.

A watermarking method of the invention optionally includes the followingsteps:

-   -   calculating a hierarchical plurality of successive levels of        motion vectors, the motion vectors of a given level each being        associated with a plurality of motion vectors of the next lower        level;    -   selecting at least some of the motion vectors belonging to the        highest level;    -   applying the watermarking function to each selected motion        vector, leading to calculating a modification parameter for said        motion vector; and    -   applying the modification parameter of the selected motion        vector to the motion vectors of a lower level associated with        said motion vector.

The hierarchical approach of this method effectively increases therobustness of watermarking by judiciously spreading the marking key overa plurality of motion vectors, starting from a single selected motionvector.

A method of the invention for watermarking a video signal may furthercomprise one or more of the following features:

-   -   the motion vectors of a given level are each equal to the        average of the motion vectors of the next lower level with which        they are associated;    -   it includes a step of calculating a hierarchy of two successive        levels of motion vectors, each motion vector of the higher level        being associated with four motion vectors of the lower level;    -   the first and second zones have substantially equal areas;    -   a sub-block centered inside the block is defined in each block        of the reference grid, the first zone being defined by the        interior of the sub-block and the second zone being the zone in        the block complementary to the first zone;    -   the blocks and sub-blocks of the reference grid are rectangular;    -   the modification, if any, applied to the selected motion vector        is a weighted symmetry;    -   the modification, if any, applied to the selected motion vector        is either a weighted central symmetry relative to one of the        vertices of the sub-block or a weighted axial symmetry relative        to one of the sides of the sub-block;    -   each bit of the binary marking key is associated with a        plurality of selected motion vectors;    -   some of the bits of the binary marking key are associated with        motion vectors calculated by motion estimation between two        images of the video signal, and at least some of the other        portion bits of the binary marking key are associated with        motion vectors calculated by motion estimation between at least        two other images of the video signal.

The invention also consists in a video signal watermarking deviceincluding means for implementing a method as described above.

The invention further consists in a computer readable data mediumincluding means for storing a video signal watermarked with the aid of amethod as described above.

The invention further consists in a method of extracting watermarkingfrom a video signal watermarked by applying a method as described above,which extraction method comprises applying a function for extracting thebinary marking key, and consisting in:

-   -   selecting the watermarked vectors;    -   marking the coordinates of each watermarked motion vector in the        reference grid;    -   assigning the binary value of the zone in which the watermarked        vector is situated to the bit of the marking key with which the        selected motion vector is associated.

A method of extracting the watermarking from a video signal may furthercomprise, for each motion vector of the highest level selected onapplying the watermarking method, the following steps:

-   -   extracting the watermarked motion vectors associated with said        motion vector;    -   calculating an average vector equal to the average of the        watermarked motion vectors associated with said motion vector;        and    -   applying the marking key extraction function to the calculated        average vector.

Finally, the invention further consists in a device for extracting thewatermarking from a video signal, which device includes means forimplementing a watermarking extraction method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the followingdescription, which is given by way of example only and with reference tothe accompanying drawings, in which:

FIG. 1 shows the successive steps of a watermarking method of theinvention;

FIG. 2 is a reference grid used for applying a watermarking function ofthe invention; and

FIGS. 3, 4, 5, and 6 illustrate rules for applying the watermarkingfunction.

DETAILED DESCRIPTION

The watermarking method shown in FIG. 1 is implemented by conventionaldata processing means, for example a microcomputer that comprises acentral processor unit (CPU) associated with random access memory (RAM)and read-only memory (ROM).

The watermarking method includes a first step 10 of motion estimation.

During this step, a source video signal S is fed to the input of themicrocomputer and a matrix 12 of motion vectors is associated with anyimage of the video signal S in the conventional way. This matrix ofmotion vectors is used to generate a prediction of the image concerned,for example starting with the preceding image of the video signal, bymoving blocks of pixels thereof as a function of the calculated motionvectors.

The motion vectors are calculated by applying a block matching method,for example. For each block of pixels of the image concerned, thismethod consists in evaluating the best motion vector for reconstructingthe current block from a block of the same size in the preceding imagedisplaced with the aid of the motion vector. To this end, a search isconducted around the current block in the preceding image in order todetermine the motion vector that minimizes a standard Displaced FrameDifference (DFD) cost function, representing the difference between thedisplaced preceding image and the current block in the image concerned.

The matrix 12 of motion vectors represented in this figure includesthirty-six motion vectors denoted {right arrow over (V)}_(i,j), with1≦i≦9 and 1≦j≦4. Of course, the number of motion vectors is generallygreater. Only thirty-six are shown to simplify the remainder of thedescription.

Then, during a step 14, a higher level of motion vectors 16 iscalculated. At this higher level, each motion vector is associated withfour motion vectors of the lower level 12. In fact, the blockscorresponding to each motion vector of the lower level 12 are groupedfour at a time into macroblocks. The higher level therefore includesnine vectors denoted {right arrow over (V₁)} to {right arrow over (V₉)}.Each motion vector {right arrow over (V_(i))} of the higher level isrepresentative of a macroblock and is calculated in the followingmanner:

${\overset{\rightarrow}{V}}_{i} = {\frac{1}{4}{\sum\limits_{j = 1}^{4}\overset{\rightarrow}{V_{i,j}}}}$

Step 14 could be iterated several times, which would create a hierarchyof motion vector levels. This example is limited to one iteration, thatis to say to two levels of motion vectors.

Then, during a step 18, a set 20 of motion vectors is selected from themotion vector 16 of the higher level in a deterministic manner or in apseudorandom manner on the basis of a secret key or on the basis of amask. In this example, the set 20 of selected motion vectors comprisesthe vectors {right arrow over (V₆)}, {right arrow over (V₅)}, {rightarrow over (V₂)}, and {right arrow over (V₉)}.

During the watermarking step 22 that follows, the selected vectors 20are modified by applying a watermarking function to them with the aid ofa marking key W. The number of motion vectors 20 selected is directlyrelated to the size of the marking key W. In this example, W is a binaryword comprising four bits, which is why four motion vectors wereselected in step 18.

Accordingly, during step 22, the parameters of the watermarking functionare set to insert the first bit of the marking key W into the motionvector {right arrow over (V₆)}, the second bit into the motion vector{right arrow over (V₅)}, the third bit into the motion vector {rightarrow over (V₂)}, and the fourth bit into the motion vector {right arrowover (V₉)}.

The chosen watermarking function is described in more detail withreference to FIGS. 2 to 6. However, generally speaking, it can bemodeled by the following equation:f({right arrow over (V _(i))})={right arrow over (V _(i))}+{right arrowover (δV _(i))}in which {right arrow over (δV_(i) )} is a parameter depending on thechosen watermarking function and on the i^(th) bit of the marking key Wto be inserted.

After this step 22, four new motion vectors {right arrow over (V′₆)},{right arrow over (V′₅)}, {right arrow over (V′₂)} and {right arrow over(V′₉)} 24 are obtained. As indicated above, these vectors satisfy thefollowing equations:

$\begin{matrix}{\overset{\rightarrow}{V_{6}^{\prime}} = {\overset{\rightarrow}{V_{6}} + \overset{\rightarrow}{\delta\; V_{6}}}} \\{\overset{\rightarrow}{V_{5}^{\prime}} = {\overset{\rightarrow}{V_{5}} + \overset{\rightarrow}{\delta\; V_{5}}}} \\{{\overset{\rightarrow}{V_{2}^{\prime}} = {\overset{\rightarrow}{V_{2}} + \overset{\rightarrow}{\delta\; V_{2}}}},} \\{{\overset{\rightarrow}{V_{9}^{\prime}} = {\overset{\rightarrow}{V_{9}} + \overset{\rightarrow}{\delta\; V_{9}}}},}\end{matrix}$

During the next step 25, the watermarking function is applied to themotion vectors of the lower level that are associated with the selectedvectors 20, using the parameters {right arrow over (δV_(i) )} calculatedduring the preceding step. For example, the transformation {right arrowover (δV₆ )} applied to the motion vector {right arrow over (V₆ )} isalso applied to the corresponding vectors of the lower level, i.e. tothe vectors {right arrow over (V_(6,1))}, {right arrow over (V_(6,2))},{right arrow over (V_(6,3))}, {right arrow over (V_(6,4))}. Thus sixteennew motion vectors 26 are obtained at the lower level, satisfying thefollowing equation:∀i∈{6;5;2;9}, ∀j∈{1,2,3,4,}{right arrow over (V′ _(i,j))}={right arrowover (δV _(i))}

These sixteen new motion vectors replace the corresponding originalvectors to furnish a new matrix 28 of motion vectors. The new matrix 28is used to obtain a watermarked version S′ of the video signal S.

In fact, during a last step 30, movement compensation is applied withthe aid of this motion vector matrix 28, based on the image that wasused as a reference for motion estimation during step 10, i.e. the imagepreceding the image concerned.

Two methods can be envisaged for this step.

A first method consists in compensating movement with the aid of all ofthe motion vectors of the matrix 28 to obtain a watermarked version ofthe image concerned.

A second method consists in compensating movement only for the blockscorresponding to the motion vectors modified by the watermarkingoperation, leaving the other blocks of the image concerned unchanged, inorder to improve image quality.

The steps 10 to 30 described above can be iterated over several imagesof the video signal S. The redundancy obtained in this way increases therobustness of the watermarking.

Watermarking redundancy can also be obtained by associating each bit ofthe marking key W with a plurality of selected motion vectors of thehigher level.

A watermarking method like that described above cannot be used in aencoding/decoding system. However, the modifications to be made to it sothat it can be used in a encoding/decoding system, using motionanalysis, will be evident to the person skilled in the art. In fact,video coding methods, such as methods using the MPEG2 standard or theMPEG4 standard, generate matrices of motion vectors to which theabove-described method can be applied when encoding the video signal.

It will be noted that step 14 is optional. Without applying this step,the selected motion vectors are the lower level motion vectors, and theless extensive spreading of the watermarking in the image can becompensated by selecting a plurality of motion vectors for each bit ofthe marking key. However, this has the effect of limiting theperformance of the watermarking in terms of invisibility. In fact, onebenefit of the hierarchical approach is applying the same deformation toadjoining motion vectors at the lower level.

In one particular embodiment of the invention, the insertion of themarking key into the video signal S can be distributed over a pluralityof pairs of images. In other words, in this case, only a portion of themarking key W is inserted into the selected motion vectors of a matrixof motion vectors calculated between two successive images, after whichthe remainder of the marking key is inserted over one or more matricesof motion vectors calculated between other pairs of successive images ofthe video signal S.

The watermarking function applied to the selected motion vectors 20 isdescribed next with reference to FIGS. 2 to 6.

Application of the watermarking function uses a reference grid that isshown in FIG. 2. This grid is placed in an orthogonal Cartesian systemof axes (O, x, y) and is made up of blocks 40 with a dimension K₀ in theabscissa direction and a dimension H₀ in the ordinate direction. In thisexample, K₀ is equal to H₀ and has a value of seven pixels.

A centered sub-block 42 with smaller dimensions is defined inside eachblock. In this example, the sub-block 42 has the dimensions K₁=H₁=fivepixels.

Two zones Z₀ and Z₁ are therefore defined within each block of thereference grid, the first zone Z₀ lying between the sub-block 42 and theblock 40 and the zone Z₁ being the interior zone of the sub-block 42.The dimensions K₀, H₀, K₁and H₁ are preferably chosen so that zones Z₀and Z₁ have substantially the same area. In this example, the total areaof each block 40 is 49 pixels, the area of zone Z₁ is 25 pixels and thearea of zone Z₀ is 24 pixels.

Let {right arrow over (V)}(V_(x),V_(y)) be a motion vector selected forapplying the watermarking function with the aid of the marking key W. Tobe more precise, applying the watermarking function to the vector {rightarrow over (V)} consists in transforming the latter so that it carriesinformation for determining the value of one of the bits of the markingkey, for example the i^(th) bit W_(i). The resulting vector is denoted{right arrow over (V′)}(V′_(x), V′_(y)).

The watermarking function is defined by applying the following rules:

-   -   if W_(i)=0 and the point V with coordinates (V_(x), V_(y)) in        (O, x, y) is in zone Z₀, then {right arrow over (V′)}={right        arrow over (V)};    -   if W_(i)=0 and the point V with coordinates (V_(x), V_(y)) in        (O, x, y) is in zone Z₁, then V is subjected to a transformation        so that the point V′ with coordinates (V′_(x), V′_(y)) in (O,        x, y) is in zone Z₀ (this transformation is described in detail        with reference to FIG. 3);    -   if W_(i)=1 and the point V with coordinates (V_(x), V_(y)) in        (O, x, y) is in zone Z₁, then {right arrow over (V′)}={right        arrow over (V)};    -   if W_(i)=1 and the point V with coordinates (V_(x), V_(y)) in        (O, x, y) is in zone Z₀, then {right arrow over (V)} is        subjected to a transformation so that the point V′ with        coordinates (V′_(x), V′_(y)) in (O, x, y) is in zone Z₁(this        transformation is described in detail with reference to FIGS. 4,        5 and 6).

FIG. 3 illustrates the situation in which W_(i)=0 and the point V is inzone Z₁, in other words inside the sub-block 42 whose vertices aredenoted A, B, C and D.

The vertex closest to V is determined, here the point A, and thenweighted central symmetry with respect to the center A is applied todisplace the point V to V′ in zone Z₀. Of course, the weightingparameter is calculated so that V′ is always in zone Z₀. For example,given the dimensions of the blocks 40 and 42, the weighted centralsymmetry is defined here by the following equation:

$\overset{\rightarrow}{{AV}^{\prime}} = {{- \frac{2}{5}}{\overset{\rightarrow}{AV}.}}$

If W_(i)=1 and V is in zone Z₀, then V′ is obtained by applying weightedaxial symmetry whose axis is the side of the sub-block 42 nearest thepoint V, if that transformation yields a point V′ in zone Z₁. This isthe situation shown in FIGS. 4 and 5.

In this case, if H denotes the projection of V onto the axis concerned,and with the dimensions chosen for this example, the weighted axialsymmetry is defined by the following equation:

$\overset{\rightarrow}{{HV}^{\prime}} = {{- \frac{5}{2}}{\overset{\rightarrow}{HV}.}}$

In the other situations represented in FIG. 6, that is to say if theimage of the point V obtained by axial symmetry as described above doesnot yield any point V′ in zone Z₁, a weighted central symmetry isapplied to the point V whose center is the nearest vertex A, B, C, or D.In the FIG. 6 example, this is the point A. Here the equation

$\overset{\rightarrow}{{AV}^{\prime}} = {{- \frac{5}{2}}\overset{\rightarrow}{AV}}$defines the weighted central symmetry. This situation arises when thepoint V is in the vicinity of one vertex of the block 40.

It is possible to detect and extract the mark inserted into a videosignal using the watermarking method described above.

The mark detection and extraction method comprises applying a functionfor extracting the binary marking key W, and consisting in:

-   -   selecting the watermarked vectors;    -   marking the coordinates of each watermarked motion vector in the        reference grid; and    -   assigning the binary value of the zone in which the watermarked        vector is situated to the bit of the marking key with which the        selected motion vector is associated.

To be more precise, if step 14 has been executed during the watermarkingof the video signal, for each motion vector 20 of the highest level 16selected during applying the watermarking method:

-   -   the watermarked motion vectors associated with that motion        vector are extracted;    -   an average vector is calculated equal to the average of the        watermarked motion vectors 26 associated with that motion        vector; and    -   the function for extracting the marking key described above is        applied to the calculated average vector.

This extraction method can extract an estimate {tilde over (W)}=({tildeover (W)}₁, {tilde over (W)}₂, {tilde over (W)}₃, {tilde over (W)}₄) ofthe marking key W=(W₁, W₂, W₃, W₄) from a watermarked image of the videosignal.

Once the marking key has been extracted, a correlation score is used todetermine a+

confidence threshold characterizing the presence or the absence of themarking key in the video signal.

A first correlation rule for estimating whether the marking key ispresent or not in an image is given by the following formula:

$C = \frac{\sum\limits_{I = 1}^{4}\;\left\lbrack {\left( {{\overset{\sim}{W}}_{i} - \overset{\_}{\overset{\sim}{W}}} \right)*\left( {W_{i} - \overset{\_}{W}} \right)} \right\rbrack}{\sqrt{\sum\limits_{i = 1}^{4}\;{\left\lbrack \left( {{\overset{\sim}{W}}_{i} - \overset{\_}{\overset{\sim}{W}}} \right)^{2} \right\rbrack*{\sum\limits_{i = 1}^{4}\;\left\lbrack \left( {W_{i} - \overset{\_}{W}} \right)^{2} \right\rbrack}}}}$${{in}\mspace{14mu}{{which}:\mspace{565mu}\overset{\_}{\overset{\sim}{W}}}} = {{\frac{1}{4}{\sum\limits_{i = 1}^{4}{{\overset{\sim}{W}}_{i}\mspace{14mu}{and}\mspace{14mu}\overset{\_}{W}}}} = {\frac{1}{4}{\sum\limits_{i = 1}^{4}{W_{i}.}}}}$

A second correlation rule for estimating the presence of the marking keyin a plurality of images I′_(n) of the signal S′ is given by thefollowing recurrent formula:

$C_{n} = \frac{{C_{n - 1}*\left( {n - 1} \right)} + \left( {1 - \frac{d\left( {{\overset{\sim}{w}}_{n},w} \right)}{8}} \right)}{n}$in which d({tilde over (w)}_(n),w) is the Hamming distance.

It is clear that a watermarking method of the invention improves therobustness and the performance in terms of invisibility of the existingmethods.

In fact, the watermarking function, which aims to place a motion vectorin a zone Z₁ or Z₀ as a function of the value of the bit of the markingkey to be inserted, effects watermarking of the video signal that ishardly visible.

Another advantage of a watermarking method of the invention lies in thehierarchical approach described above with respect to step 14 inparticular of the above-described method. Spreading the watermarkingover a plurality of motion vectors of a lower hierarchical level basedon the selection of a corresponding motion vector from a higherhierarchical level reduces the effect of an attack on the watermarkingof the video signal, whether malicious or not.

1. A method of watermarking a video signal by applying a watermarkingfunction to motion vectors calculated by estimation of movement betweenimages of the video signal, the method comprising the following steps:applying the watermarking function to at least some of the calculatedmotion vectors; and generating the watermarked video signal bycompensating movement with the aid of the watermarked motion vectors,the watermarking function being applied with the aid of a binary markingkey, each bit of which is associated with at least one selected motionvector, wherein, in order to apply the watermarking function, the methodfurther comprises the following steps: marking a point (V) correspondingto the coordinates (Vx, Vy) of the selected motion vector ({right arrowover (V)}), the marking taking place in a reference space (O, x, y)divided into a plurality of predetermined portions; defining twocomplementary zones Z₀ and Z₁ in each portion, one of the two zonesbeing situated inside the other one; assigning a binary value to each ofthe two zones; and if the point corresponding to the coordinates of theselected motion vector is in the zone of the portion to which itbelongs, of binary value which corresponds to the bit of the marking keywith which the selected motion vector is associated, not modifying thecoordinates of the selected motion vector; if the point corresponding tothe coordinates of the selected motion vector is not in the zone of theportion to which it belongs, of binary value which corresponds to thebit of the marking key with which the selected motion vector isassociated, modifying the coordinates of the selected motion vector sothat the point is in the zone, of binary value which corresponds to thebit of the marking key with which the selected motion vector isassociated.
 2. The method according to claim 1 for watermarking a videosignal, wherein the reference space is a reference grid including blockswith predefined dimensions, each block including first and second zones.3. The method according to claim 1 for watermarking a video signal, themethod including the following steps: calculating a hierarchicalplurality of successive levels of motion vectors, the motion vectors ofa given level each being associated with a plurality of motion vectorsof the next lower level; selecting at least some of the motion vectorsbelonging to the highest level; applying the watermarking function toeach selected motion vector, leading to calculating a modificationparameter for said motion vector; and applying the modificationparameter of the selected motion vector to the motion vectors of a lowerlevel associated with said motion vector.
 4. The method according toclaim 3 for watermarking a video signal, wherein the motion vectors of agiven level are each equal to the average of the motion vectors of thenext lower level with which they are associated.
 5. The method accordingto claim 3 for watermarking a video signal, the method including a stepof calculating a hierarchy of two successive levels of motion vectors,each motion vector of the higher level being associated with four motionvectors of the lower level.
 6. The method according to claim 1 forwatermarking a video signal, wherein the first and second zones havesubstantially equal areas.
 7. The method according to claim 2 forwatermarking a video signal, wherein a sub-block centered inside theblock is defined in each block of the reference grid, the first zonebeing defined by the interior of the sub-block and the second zone beingthe zone in the block complementary to the first zone.
 8. The methodaccording to claim 7 for watermarking a video signal, wherein the blocksand sub-blocks of the reference grid are rectangular.
 9. The methodaccording to claim 1 for watermarking a video signal, wherein themodification of the coordinates of the selected motion vector, if any,is a weighted symmetry applied to the point (V).
 10. The methodaccording to claim 8 for watermarking a video signal, wherein themodification of the coordinates of the selected motion vector, if any,is either a weighted central symmetry relative to one of the vertices ofthe sub-block or a weighted axial symmetry relative to one of the sidesof the sub-block applied to the point (V).
 11. The method according toclaim 1 for watermarking a video signal, wherein each bit of the binarymarking key is associated with a plurality of selected motion vectors.12. The method according to claim 1 for watermarking a video signal,wherein some of the bits of the binary marking key are associated withmotion vectors calculated by motion estimation between two images of thevideo signal, and wherein at least one other portion of the bits of thebinary marking key is associated with motion vectors calculated bymotion estimation between at least two other images of the video signal.13. A device for watermarking a video signal, the device including meansfor implementing the method according to claim
 1. 14. A computerreadable data medium, including means for storing a video signalwatermarked with the aid of the method according to claim
 1. 15. Amethod of extracting watermarking from a video signal watermarked byapplying the method according to claim 1, which extraction methodcomprises applying a function for extracting the binary marking keycomprising: selecting the watermarked vectors; marking a pointcorresponding to the coordinates of each watermarked motion vector, themarking taking place in the reference space; and assigning the binaryvalue of the zone in which the point is situated to the bit of themarking key with which the selected motion vector is associated.
 16. Themethod according to claim 15 for extracting the watermarking from thevideo signal, the video signal further watermarked by the followingsteps: calculating a hierarchical plurality of successive levels ofmotion vectors, the motion vectors of a given level each beingassociated with a plurality of motion vectors of the next lower level;selecting at least some of the motion vectors belonging to the highestlevel; applying the watermarking function to each selected motionvector, leading to calculating a modification parameter for said motionvector; and applying the modification parameter of the selected motionvector to the motion vectors of a lower level associated with saidmotion vector; wherein, for each motion vector of the highest levelselected on application of the watermarking method, the following stepsare applied: extracting the watermarked motion vectors associated withsaid motion vector; calculating an average vector equal to the averageof the watermarked motion vectors associated with said motion vector;and applying the marking key extraction function to the calculatedaverage vector.
 17. A device for extracting the watermarking from avideo signal, the device including means for implementing the methodaccording to claim
 15. 18. The method according to claim 9, forwatermarking a video signal, wherein the modification of the coordinatesof the selected motion vector, if any, is either a weighted centralsymmetry relative to one of the vertices of the sub-block or a weightedaxial symmetry relative to one of the sides of the sub-block applied tothe point (V).